The invention relates to the printing of indicating codes or marks which vary in colour as a function of temperature over time (time-temperature indicator). The practice of labelling foodstuffs with best before, use by or sale by dates is widespread around the world. In many regions the use of such codes is driven by legislation that is designed to ensure that food is safe to consume. The choice of use by date is generally made using a conservative set of assumptions based around worst case scenarios for storage temperature and shipping times in the supply chain. The use-by date on packaging does not inform about a particular state of the food, but simply reports the date when food stored in worst case conditions is not safe to eat. It follows, therefore, that a large quantity of food is discarded when it is still safe to eat. Best before dates are also chosen with no knowledge of how a consumer will transport or store the food that they have purchased. The use of these codes again leads to food being discarded because it has spoiled before the expiry of the code.
During storage and transit, temperature is one of the most important factors influencing the rate of microbial growth in and the amount of physical and chemical deterioration in food products. It is therefore useful to report the time-temperature history along the cold chain, so that the consumer can make an informed choice about the food products they choose, and the suppliers of food products can use a less conservative set of assumptions in setting use-by dates. A model time-temperature indicator would be a simple, cost-effective and provide an easily understood indication that the temperature and time history of the food product means that it is of good quality and safe to consume.
A common approach to producing time-temperature indicator technology is to make use of a reversible photo-activated reaction that yields a colour change upon activation. Such materials have an initial colour, which is changed when activated with light of a suitable wavelength and then revert to their original colour over time. The rate of change of colour of the photo-activated compound in such indicators can be influenced by exposed temperature. However, the use of reversible photo-activated reactions as the basis of indicating codes or marks can lead to reactivation/resetting of the code by re-exposing the code or mark to light of a suitable wavelength. In many cases recharging of the colour is easily achieved by ambient light, even after initial activation by UV or after fading of the colour of the photo-chromic species stored at high temperature. It is, therefore, important to protect the activated time-temperature indicating code from further activation, in order to protect the integrity of the code.
A variety of approaches have been used to address this problem within the state of the art. U.S. Pat. No. 8,007,900 discloses time-temperature indicators protected from additional light exposure by a UV-Vis filter (a light absorbing layer) applied by thermal transfer or inkjet printing immediately after the activation of the photo-chromic colorant. The light absorbing layer contains a binder and ultra-violet absorbers.
WO 2005/075978 relates to a photo-chromic colorant introduced into or atop a support matrix. The time temperature indicator is then optionally provided with a protector, which prevents the renewed photo-induced coloration of the reversible indicator. The protector may be a protective coating or a laminate that comprises a filter, for example a polypropylene colour filter, which filters out certain wavelength ranges.
U.S. Pat. No. 8,403,558 discloses a time-temperature indicator comprising one layer containing a photo-chromic colorant into a matrix substrate, and a transparent colourless or transparent coloured light absorbing layer applied atop. A UV light absorber, a polymeric binder and other additives, such as optical brighteners, form the light absorbing layer.
It can be readily understood that all of the solutions proposed so far involve the use of a light absorbing layer that is either applied or printed atop the layer containing the photo-chromic colorant. The application of a top layer is costly, time-consuming and requires the complex set-up of a production line, involving the co-ordination of several pieces of in-line processing equipment.
Accordingly, it would be desirable to provide an improved inkjet ink and an improved time temperature indicator (TTI). In particular it would be desirable to provide a simpler and more economical time temperature indicator.
It is one object of the present invention to overcome or address the problems of prior art inks and TTI's or to at least provide commercially useful alternatives thereto. It is an alternative and/or additional object to provide an inkjet ink and TTI which is cheaper to make and/or more effective than known inkjet inks/TTI's.